1. Field of the Invention
The present invention relates to an electric resistance type foil strain gauge, and more particularly to a foil strain gauge used in a strain gauge type load cell or various types of strain transducers.
2. Description of the Related Art
FIG. 1 is a plan view showing the configuration of a conventional electric resistance type foil strain gauge.
The conventional electric resistance type foil strain gauge (hereinafter referred to as foil strain gauge) measures a strain by applying a physical phenomenon that the resistance of an electric resistor changes when a strain is applied to the electric resistor. The foil strain gauge is attached to the surface of an object to be measured or is buried therein, detects a strain and transduces it into a quantity of electricity
As shown in FIG. 1, the conventional foil strain gauge has a plurality of sensing portions 21 arranged in a loop.
FIG. 2 is an enlarged plan view of part of the conventional foil strain gauge. The foil strain gauge includes a plurality of turnup tabs 22 in addition to the plurality of sensing portions 21. The sensing portions 21 and the turnup tabs 22 are connected to each other through turnup portions 23. The turnup portions 23 each have a semicircular inner side R with a constant curvature.
When stretching force acts on the foil strain gauge, a stress is concentrated on the inner side R of each of the turnup portions 23. This is because stress concentration occurs near a place where the foil strain gauge has its width changing discontinuously. The stress concentration appears clearly when a line width of the metal resistor changes sharply or the ratio of a length L of the turnup tab 22 to a line width W of the sensing portion 21 is large. The stress concentration causes the metal resistor to crack, and the foil strain gauge reaches the end of its fatigue service life.
A fatigue test was performed on the conventional foil strain gauge with an alternating load of 1,500 μst based on NAS (National Aerospace Standard) 942. The result of the fatigue test shows that cracks were formed in the turnup portions 23 of the metal resistor, and that the foil strain gauge had a short fatigue service life.
FIG. 3 shows a result obtained by subjecting the conventional foil strain gauge to modeling analysis. The turnup portions 23 are caused to crack by the stress concentrated thereon. According to a modeling result obtained by finite element method analysis using three-dimensional CAD, it is confirmed that stress was concentrated on the turnup portion as shown in FIG. 3.
It is known that, in the conventional foil strain gauge, it is possible to adjust by the length L of the turnup tab 22 the so-called creep characteristics that the output value fluctuates with the lapse of time when a predetermined load is kept applied. When a long turnup tab is required in the conventional foil strain gauge with the turnup portion 23 having the semicircular inner side, the creep characteristics can be adjusted, but the fatigue service life is shortened as described above.
FIG. 4 shows another conventional foil strain gauge.
For example, this is a foil strain gauge with a new pattern, which is described in the Journal of Japan Society of Mechanical Engineers, Vol. 77, No. 668, July 1974. In this foil strain gauge, loop-like dynamic approach sections 35 are formed at the ends of sensing portions 31 to improve the service life.
In this conventional foil strain gauge, however, since the pattern size increases, the material cannot be used economically, thereby increasing the manufacturing cost.